Cavity filters and their uses are well known in the electrical filter art. A cavity filter is basically a tuned electrical filter having a resonant frequency that is determined, in part, by the geometry of a cavity. The cavity may house component(s), such as a helical coil or rod, for example. Openings in the case surrounding the cavity may be used to affect the resonant frequency of the cavity filter. The cavity and the component(s) housed in the cavity (if any) create capacitance and inductance values that determine the resonant frequency of the cavity filter.
Cavity filters normally include a tuning apparatus that permits a user to precisely tune the filter to a nominal resonant frequency. Such an adjustment may be required to compensate for manufacturing tolerances and/or variations in materials used to build the cavity filter, which can cause the actual resonant frequency to vary from the nominal resonant frequency. One form of tuning apparatus is a tuning screw. Tuning screws are usually in the form of a threaded slug that is screwed into the cavity of the filter through a threaded hole. The amount that the tuning screw penetrates into the cavity controls the capacitance or the inductance values of the filter. Consequently, the resonant frequency of the cavity filter can be changed by changing the penetration of the tuning screw.
One problem associated with cavity filters is their sensitivity to temperature changes. Changes in temperature produce physical changes in cavity geometry that, in turn, produce changes in the electrical characteristics of the cavity filter. These capacitance and/or inductance changes cause the actual resonant frequency of the cavity filter to "drift" from the nominal resonant frequency. In many applications, cavity filters are required to operate over a wide temperature range. In such applications, temperature-induced frequency drift can be significant enough to make a cavity filter ineffective. For example, a significant lowering of the resonant frequency due to increasing temperatures will cause a corresponding lowering of the stopband cutoff frequency of a cavity filter. As a result, important signal information may be inadvertently filtered out by the cavity filter.
One approach that has been adopted by cavity filter manufacturers to correct the temperature-induced frequency drift problem is the use of temperature-stable materials in the construction of cavity filters. Cavity filters built with materials having low thermal expansion characteristics are less sensitive to temperature change than cavity filters built from other materials. However, when exposed to large temperature changes, even these cavity filters are subject to frequency drift problems, albeit to a lesser degree. In applications where a cavity filter forms a part of signal transmitting and receiving equipment operating with high frequency signals, even reduced frequency drift can adversely affect the sensitivity of the equipment or render the equipment totally useless.
Unfortunately, the tuning screws previously used in cavity filters are not effective in correcting temperature-induced frequency drift because prior art tuning screws react similarly to temperature changes as do the components of a cavity filter, i.e., prior art tuning screws expand and contract in response to increasing and decreasing temperatures in the same way that other cavity filter components respond to increasing and decreasing temperatures. As a result, prior art tuning screws may contribute to the temperature-induced frequency drift problem, rather than provide a solution to the problem.
As will be appreciated from the foregoing discussion, there has developed a need in the electrical filtering art for a cavity filter whose performance is less sensitive to temperature change over a wide range of temperatures. The present invention provides a temperature-compensated tuning screw that, when used with prior art cavity filters, actively compensates for temperature-induced changes in the cavity filter and thereby reduces the amount of temperature-induced drift in the resonant frequency of the cavity filter.